Nickel-chromium-iron alloy

ABSTRACT

A STAINLESS STEEL ADAPTED FOR USE AT HIGH TEMPERATURES CONTAINS FROM UP TO 4% BY WEIGHT OF TUNGSTEN, TOGETHER WITH TITANIUM AND ALUMINUM.

July 30, 1974 sbp ETAL 3,826,649

NICKELCHROMIUM-IRON ALLOY Filed Dec. 8, 1972 Fig.7

"Allo 000" 0.5-

Fig.2

United States Patent 3,826,649 NICKEL-CHROMIUM-IRON ALLOY Rolf HaraldSiiderberg and Clas Erik Helmer, Sandviken, Sweden, assignors to SandvikAktiebolag, Sandviken, Sweden Filed Dec. 8, 1972, Ser. No. 313,322Claims priority, application Sweden, Dec. 21, 1971, 16,378/71 Int. Cl.C22c 37/10, 39/02 US. Cl. 75-124 9 Claims ABSTRACT OF THE DISCLOSURE Astainless steel adapted for use at high temperatures contains from up to4% by weight of tungsten, together with titanium and aluminum.

The present invention relates to a Ni-Cr-Fe alloy for use at hightemperatures.

Alloys of the type 20%. CR-30% Ni and the rest substantially Fe are usedi.e. in the petrochemical and the hydrocarbon processing industriesparticularly in tubes and furnace details.

It has been tried to fulfill the always increased demands upon creeprupture strength and heat resistance of such alloys by i.e. additionalalloy components or higher alloying additions.

Thus, it is earlier known that an addition of W by solution hardeningincreases the creep rupture strength in alloys of 20 Cr-30 Ni base. Itwas found among other things, by means of creep testing at relativelyshort times (up to about 1000 hours) that the creep rupture strength at900 C. increased particularly much at contents above 4% W.

Based upon these results, alloys with relatively high W contents andincreased contents of mainly Ni and Co have been made, said alloysshowing improved creep rupture strength compared to the prior 20 Cr-30Ni base alloys.

A disadvantage with these newer alloys has been, however, the relativelyhigh costs because of the increased alloying contents and the relativelycomplicated manufacturing.

Another method which has been used to improve the creep rupture strengthof the 20 Co-30 Ni base alloys has been to increase the C content,sometimes in combination with an increased Ti content. This method toimprove the creep rupture strength is relatively cheap and gives aconsiderable increase in creep rupture strength for short service times.A disadvantage with this type of alloys, however, is that for prolongedservice times the creep rupture strength will decrease rapidly, at leastat the higher service temperatures which are normally of interest,because of overaging of the carbide particles. Thus, for the servicetimes normally required there will only be an insignificant improvementin creep rupture strength.

According to the invention it has been found possible by adding onlyabout 2-4% W to obtain at least the same creep rupture strength as forthose alloys which, based upon solid solution hardening, contain highercontents of W, Ni and Co. The reason for this is probably that thehigher contents of Ni and Co, added to improve the solubility of W willreduce the creep rupture strength by increasing the mobility of thedislocations. By adding at the most 3.5% and preferably at the lowest2.5% W to the 20 Co-30 Ni base alloys it was also found that the creeprupture strength was improved but no reduction in creep ductility wasobserved.

The invention also includes a combination as an addition of W and anincrease of the contents of C and Ti. This variant of the new alloy canbe used when a still higher creep rupture strength is needed and whensome reduction in creep ductility can be tolerated. The surprisingproperty of this alloy is that the further increase of the creep rupturestrength, obtained by increasing the contents of C and Ti, will notdecrease at increased service times, contrary to the earlier knownalloys which had an increased content of C and Ti but no addition of W.Obviously, by some mechanism the W addition prevented or at leastreduced the rate of the overaging of the carbide particles. The Waddition in the high C+Ti grade thus increases the creep rupturestrength in two ways, by solid solution hardening and by counteractingthe overaging of the carbide particles.

The alloy according to the invention contains, in percent by weight,from traces up to 0.40% C, 20-35% Ni, 15-25% Cr, 2-4% W, 0.2-1.6%"Ti,0.2-1.0% A1, at the most 1% Si, at the most 3% Mn, 0-0.1% B and the restFe besides normally present impurities. In addition to good heatresistance and high creep rupture strength the alloy is characterized bygood ductility and absence of embrittling phases in any appreciabledegree.

The optimum W content has been found to be about 3% for servicetemperatures around 900 C. At higher contents of W, an improved creeprupture strength will be obtained at short service times, but atprolonged service times the creep rupture strength will be reducedcompared to the alloy containing 3% W because of precipitation ofembrittling phases. At service-temperatures above 1000 C. the W contentcan be increased to about 4%. A lower W content than 3% will usually notgive a complete solid solution hardening.

In the low C grade of the new alloy the C content is at the most 0.15%and at the lowest 0.05% which is in the same order as for ordinary Alloy800. At this carbon level the creep ductility will be approximately thesame as for Alloy 800. At this carbon level the solution heat treatmenttemperature should be around 1150 C., to give a complete resolution ofthe TiC particles.

At higher C contents than about 0.15%, and preferably at C contents ofat the most 0.35% and at the lowest 0.20%, the solution heat treatmenttemperature must be increased for the solution of the TiC to obtainmaximum creep rupture strength. Thus, at a C content of about 0.2% thesolution heat treatment temperature should be around 1250" C.

Ti should preferably be added in stoichiometric proportion to the Ccontent, thus about 4X C, to obtain maximum creep rupture strength andductility.

A corresponding effect regarding solution hardening and oxidationresistance may also be obtained by the addition of A1. A particularlysuitable content has been 03-08%.

Concerning the main alloying elements Ni and Cr, principally nickelcontributes to a stable austenitic structure while chromium mainly givesthe alloy good oxidation resistance and high resistance tocarburization. Lower Ni contents than about 20% are generally not usedbecause of risks of instability and formation of embrittling sigmaphase. At moderate demands upon oxidation resistance, however, it hasbeen found sufficient to have 2. Ni content of -25%. At the high demandsnormally present, a Ni content of at least must be used. The content ofCr must be well adjusted in consideration of the fact that an increasedcontent gives unfavourable sigma phase formation. An optimum content, inregard of oxidation resistance as well as of sigma phase formation, hasshown to be at the lowest 19% and preferably at the most 23% Cr.

Silicon and manganese may be present in contents normal for this kind ofalloys. Particularly Si has shown a favourable effect Concerning theoxidation resistance. Each alloying element should be present incontents of at the lowest 0.3%. Mn should preferably be at the most1.5%.

Concerning normally occurring impurities, S and P may usually be presentin contents of at the most each 0.015%. The contents of other possiblealloying elements should also be low and be regarded more as impurities.Niobium has thus shown a less stabilizing effect than Ti and tendenciesto form embrittling phases have been found. A normal impurity content ismax. 0.1% Nb. Cobalt is of small interest as addition in alloysaccording to the invention and neither positive nor negative effectshave been found. Because of the high costs of this alloying element thecontents should thus be as low as possible. A normal impurity content ismax. 0.1% Co.

It is per se known that B in small amounts can improve the creepstrength of alloys of actual type. Furthermore, it improves theductility in the hot working range and influences the carbideprecipitation favourably. A suitable content has shown to be max. 0.005%B.

The alloy according to the invention is particularly useful formanufacturing plastically worked products as for instance tubing, bar,plate, forgings, etc.

As has been mentioned earlier the petrochemical and the hydrocarbonprocessing industries are important consumuers. The alloy according tothe invention has here shown particularly superior properties at use intubes for steam reforming of hydrocarbon, in which process hydrogen andcarbon monoxide are formed. The mateial has found similar use in socalled pig tail tubes and as tubing in ethylene furnaces.

Characteristic for the alloy according to the invention has been theparticularly high resistance to creep at temperatures above 700 C. andpreferably also above 800 C. under heavy mechanical stress and verycorrosive conditions. It has also been found that the alloy has the samegood resistance to thermal fatigue and high ductility as the base alloyswith 20 Cr- Ni, i.e. Alloy 800.

The following example shows the results obtained in creep testing alloysaccording to the invention. The alloys (No. l and No. 2) were comparedwith the standard alloy, Alloy 800, on 20 Cr-3O Ni base. The compositionof the test material is evident from Table I.

Results from creep testing at 900 C. have been assorted in Table II. Thecreep test results are also illustrated in diagram, see FIG. 1, showingthe connection between the stress (a) in kp./mm. and the time tofracture (t) in hours.

TABLE II Results of creep testing at 900 0.

Percent Stress, Time to Alloy kp./ rupture, Area No. Heat treating mm.hrs. Elonga. reduction 1 Solution 1,150 0., 4. 0 255 39. 7 62 30 min.quenching, H20. 1 do 3. 0 1, 006 50. 3 57 l do 2. 5 2,125 26. 4 33 1 do2.0 5, 616 45. 1 36 1 do 1. 5 1 Solution 1,250 0., 4 564 15.4 14

' 30 13in. quenching,

I 2 do 3 1, 754 20. 7 15 2 do 2. 5 9, 637 12.3 I. 8 2 do. 2 2 .do 1. 5

1 Still running.

In FIG. 2 the creep rupture strength (0 in kp./mm. at 900 C. at failureafter 100,000 hours is given as a function of the tungsten content (W%).The test materials were a 20 Cr-30 Ni base alloy with varying additionsof W (x-marked points) and alloy No. 1 (o-marked point).

From the results it is evident that an addition of only about 3% W atlong times (above 10,000 h.) has given the highest creep rupturestrength. It has thus been possible to obtain about higher creep rupturestrength (for 100,000 h. at 900 C.) compared to the 20 Cr-30 Ni basealloy at a very moderate increase of the alloying content (i.e. theprice). The low carbon alloy according to the invention (N0. 1) hasshown about the same creep ruptpre strength as considerably moreexpensive and complicated alloys.

The results also show that the high carbon variant (No. 2) has obtained45% higher creep rupture strength than the low carbon variant (No. l).The ductility of No.

2 is naturally lower, but quite acceptable. A necessary condition forsufficient ductility should be an addition of Ti in stoichiometricproportion.

We claim:

1. Nickel-chromium-iron alloy having excellent heat resistance and highcreep rupture strength in combination with good ductility at long timeuse, said alloy consistin'g, in percent by weight, from traces up to0.40% C, 20-35% Ni, 15-25% Cr, 24% W, 0.21.6% Ti, 0.2- 1.0% A1, 0.3-1.0%Si, 0.33.0% Mn, 00.01% B, and the balance Fe.

2. Alloy according to claim 1 wherein the Ti content is about 4 timesthe content of C.

3. Alloy according to claim 1 wherein the A1 content is 0.30.8%.

TABLE I.CHEMICAL ANALYSES OF TEST MATERIAL Alloy No. C Si Mn Cr Ni W TiAl B Fe 1* 0. 11 0. 60 0. 55 20. 5 29. 9 2. 95 0. 45 0. 23 0. 005 Rest.2* 0. 21 0. 58 0. 55 21. 7 30. 9 3. 14 0. 92 0. 28 0. 011 Do. Alloy800** 0. 05 O. 55 0.55 21.0 31. 0 0. 35 0. 30 Do.

Alloys according to the invention (with low resp. high content ofcarbon).

"Reference material (nominal analysis).

4. Alloy according to claim 1, wherein the W content is 2.5-3.5

5. Alloy according to claim 1, wherein the C content is 0.05-0.15%.

6. Alloy according to claim 1, wherein the Ni content is 20-35%.

7. Alloy according to claim 1, wherein the Cr content iS 8. Alloyaccording to claim 1, wherein the Mn content is 0.31.5%.

5 6 9. Alloy according to claim 1, wherein the C content 3,212,88410/1965 Soler 75-124 is 0.200.35%. 3,243,287 3/1966 Lillys 7S-l24References Cited UNITED STATES PATENTS HYLAND BIZOT, Primary Examiner2,143,423 1/1939 Remmers 7s 124 5 US. 01. m.

3,169,858 2/1965 Heydt 75-124 75-123 128 W

